OJS currently handles all files in the native format from which they're supplied; there is no conversion done. However, our Lemon8-XML
software does approach this issue by converting articles from most layout formats (unfortunately not LaTeX due to poor support by word processors) to OpenDocument ODT, and from there into a particular XML schema as appropriate. We are planning some major development centred around converting XML into PDF for article publication and display, but that is a much larger effort.
Your best bet might to be to simply use one of the numerous TeX-to-PDF converters (most of which are free or open source) available from the web to convert submissions once they're uploaded into PDF for the review or layout stage, depending on your needs. This is a bit more editorial work, but the simplest alternative at present.